The present invention relates generally to the field of medical imaging and more particularly to a method and apparatus for timing calibration in a PET scanner.
Positron emission tomography (PET) is a technique commonly used in clinical medicine and biomedical research to create images of a living body in its active state. PET scanners can produce images that illustrate various biological process and functions. In a PET scan, the patient is initially injected with a radioactive substance known as a PET isotope. The injected PET isotope can act as a tracer when it becomes involved in certain physiological processes in the patient's body. Typical positron-emitting PET isotopes include 11C, 13N, 15O and 18F. When positrons are emitted within the body, they combine with electrons in the neighboring tissues and become annihilated. The annihilation event mostly result in a pair of gamma photons being released in opposite directions. The gamma photons are then detected by a detector ring like the one shown in FIG. 1. The detector ring 100 may comprise a number of detectors or detector channels (e.g., 11, 12, 13, and 14 etc.) each having a scintillator block and a photomultiplier tube (PMT). For example, detector 11 comprises a scintillator block 112 and a PMT 114, detector 12 comprises a scintillator block 122 and a PMT 124, and so on.
In operation, a patient 102, who has been injected with a PET isotope, may be positioned in the detector ring 100. One pair of gamma photons from a body part 104 may be detected along a line of response (LOR) 116 on opposite sides of the patient, for example. Another pair of gamma photons from the body part 104 may be detected along another LOR 136. Along the LOR 116, the gamma photons may cause substantially simultaneous scintillations in the scintillator blocks 112 and 122. These scintillations may then be amplified and converted into electrical signals by the PMTs 114 and 124 respectively. Subsequent electronic circuitry may determine whether these substantially simultaneous scintillations are coincidence events, that is, radiation events originating from the same annihilation event in the patient 102's body. Data associated with coincidence events along a number of LORs may be collected and further processed to reconstruct two-dimensional (2-D) tomographyic images. Some modern PET scanners can operate in a three-dimensional (3-D) mode, where coincidence events from different detector rings positioned along the axial direction are counted to obtain 3-D tomographic images. An exemplary PET scanner with multiple detector rings is shown in FIG. 2, where the PMTs are not shown. As shown, the PET scanner 200 comprises three detector rings 22, 24 and 26.
One aspect of PET detection methods is Time-Of-Flight PET (TOF PET), where the arrival time of a pair of coincident photons is measured. In non-TOF PET, the arrival time is ignored and the annihilation is equally probable to have occurred along the full extension of the LOR. In TOF PET, upon detection of a radiation event (e.g., a gamma photon), the scintillator block at the detection locale time-stamps the detected radiation event. Incorporation of the arrival time gives more weight to the more probable locations of the emission point for each event, thereby reducing statistical uncertainty in the reconstructed images. For the TOF PET technique to be successful, the PET scanner has to maintain a high timing resolution (e.g., within a fraction of a nanosecond). The timing resolution greatly depends on how two scintillator blocks on opposite sides of the detector ring time-stamp their respectively detected radiation events. If the two opposite blocks have different timing characteristics, radiation events detected along the LOR connecting the two blocks may exhibit a timing drift, therefore causing difficulty in reconstructing PET images or causing errors in the reconstructed images. Timing drifts are typically caused by changes in time delay inside the PMTs, change in thresholds in the time-stamping circuits, master clock skew, or transmission delays in the cables. Any of these changes can be caused by either actual component change or thermal effects.
Due to the timing resolution requirements, it is typically necessary to monitor and calibrate the timing drift for a PET scanner on a frequent basis. This type of timing calibration is traditionally performed by placing a radiation source at the center of the detector rings and adjusting each detector channel until the coincidence events registered by each channel is maximized and relatively uniform around the entire ring. The use of an external radiation source requires interruptions to normal operation of the PET scanner. As a result, the traditional timing calibration cannot be performed concurrently with data acquisition from patients.
In view of the foregoing, it would be desirable to provide a more efficient solution for timing calibration in a PET scanner.